Hello lovely writers. I have been planning out this fantasy book series for about half a year, and mainly want it to appeal to children ages 10+ (I say mainly because I hope it will appeal to readers of all ages, although that may be a stretch). The main problem I foresee is that my writing style tends to have long, complex sentences, which certainly wouldn't work for the younger kids. I know there also a lot of things that need to or should be changed to appeal to children.

That last point is tricky. Some books only appeal to a younger audience because they skim the surface of emotional issues, while others have embedded multiple layers of emotion into their story that can be understood by a wide variety of ages. It's really up to how "young" you want to make the series.

A writer is a world trapped in a person— Victor Hugo

Ink is blood. Paper is bandages. The wounded press books to their heart to know they're not alone.

Find some really great books that have already been published for your target group and read them. Take note of what you think they did well or didn't do well, what you think worked or didn't work, what you liked or didn't like, and try to pinpoint why. Then try to incorporate this knowledge into your own writing. Obviously don't copy what others have done but getting a feel for what is already there and what has or hasn't worked can give you an idea of what you should or shouldn't do.